This invention relates to a solid laser device which efficiently performs excitation using a semiconductor laser, and more particularly to means for introducing a pumping light beam from a semiconductor laser to a solid laser medium with high efficiency.
FIG. 1 is an explanatory diagram showing the arrangement of a semiconductor laser excited solid laser device disclosed by U.S. Pat. No. 3,624,545, patented on Nov. 3, 1971.
In FIG. 1, reference numeral 1 designates a solid laser medium; 2, a semiconductor laser array; 3, a reflecting cylinder; 4, a high reflection mirror; 5, an output coupling mirror; 6, an optical axis; and 7, a laser resonator mode. That is, FIG. 1 shows a so-called "side light excited solid laser device" in which the solid laser medium 1 is excited by light emergent from the semiconductor laser array 2 which is an array of a number of semiconductor lasers having a light emission spectrum substantially equal to the absorption spectrum of the solid laser medium 1.
The output light of the semiconductor laser array 2 is applied to the solid laser medium 1 in such a manner that it is substantially perpendicular to the side of the latter. A part of the light is absorbed by the solid laser medium 1, and the remaining passes through the solid laser medium 1, and reflected by the reflecting cylinder 3, so that it is applied to the solid laser medium 1 again, thus forming an excitation region permitting laser amplification. The high reflection mirror 4 and the output coupling mirror 5 form a laser resonator, thus providing the laser resonator mode 7. The excitation region is not particularly related to the mode distribution of the laser resonator mode 7; that is, it is lo provided substantially throughout the solid laser medium 1. It is preferable to select the laser oscillation TEM.sub.oo mode (Transverse ElectroMagnetic mode) which is the fundamental of the laser resonator mode 7. FIG. 2 is a sectional view taken across the optical axis 6 in FIG. 1, showing the relation between the excitation region permitting laser amplification and the laser resonator mode 7. In FIG. 2, reference numeral 8 designates the excitation region. Almost all the excitation energy accompanying the output light of the semiconductor laser array 2 is applied to the region of the solid laser medium 1 except that occupied by the laser resonator mode 7; that is, a larger part of the excitation energy does not concern the amplification of the laser oscillation mode, with the result that the excitation is low in efficiency.
In order to eliminate the difficulty that the excitation region and the laser oscillation mode are greatly different from each other in size or in volume, U.S. Pat. No. 4,653,056 patented on Mar. 24, 1987 has disclosed an end (face) light excited solid laser in which the emergent light of an exciting light source, namely, a semiconductor laser is arranged substantially in parallel with the optical axis 6 of the laser resonator described above, so that excitation is made from the end face of the solid laser medium 1 which is substantially perpendicular to the optical axis 6. The arrangement of the solid laser disclosed by the U.S. Pat. No. 4,653,056 is as shown in FIG. 3. In FIG. 3, reference numeral 9 designates a semiconductor laser; 10, a lens; and 11, a pumping light beam. The divergent emergent light of the semiconductor laser 9 is converted by the lens 10 into a convergent light beam. The convergent light beam, after passing through the high reflection mirror 4, is applied to the solid laser medium 1 through one of the end faces which are substantially perpendicular to the optical axis 6. In order to increase the efficiency of excitation, the excitation region is matched with match the mode volume of the fundamental mode TEM.sub.oo of the laser oscillation mode 7. The end light excited solid laser using semiconductor laser is high in the efficiency of excitation, because the excitation can be matched with the mode volume of the fundamental mode TEM.sub.oo. However, the output of the solid laser is limited, because almost all the conventional semiconductor lasers are limited to about 1 W in output, and even if a higher output semiconductor laser is employed as pumping light source, the energy usable for the end light excited solid laser is limited. On the other hand, the side light excited solid laser can introduce much more energy from the pumping light source into the solid laser medium; however, it suffers from the matching of the excitation region with the mode volume of the TEM.sub.oo mode.
In order to overcome this difficulty, U.S. Pat. No. 4,710,940 patented on Dec. 1, 1987 has disclosed a solid laser in which, a pumping light source, namely, a semiconductor laser array is of a side light excited type, but its laser resonator is changed in arrangement so that the output light of the semiconductor laser array coincides substantially with the optical axis of the laser resonator.
The solid laser disclosed by the U.S. Pat. No. 4,710,940 is as shown in FIG. 4. In FIG. 4, reference numeral 12 designates a trapezoid solid laser medium having first and second side faces 14 and 15; 13, a first end face; and 16, a second end face. Multi-layer films of dielectric are formed on the first and second side faces 14 and 15 which reflect the wavelength of oscillation of the solid laser at high percentage, but which reflect the wavelength of oscillation of the semiconductor laser at low percentage 19 which is the pumping light source. Similarly, multi-layer films of dielectric are formed on the first and second end faces 13 and 16 which reflect the wavelength of oscillation of the solid laser at low percentage. In the solid laser thus constructed, the laser beam in the laser resonator, as shown in FIG. 4, advances zigzag in the solid laser medium 12 while being reflected between the first and second side faces 14 and 15. On the other hand, the output light of the semiconductor laser, which is the pumping light source, is obliquely applied to the first side face 14 or the second side face 15 in such a manner that its optical axis coincides substantially with that of the laser beam advancing zigzag. Therefore, similarly as in the case of the end light excited solid laser described with reference to FIG. 3, the excitation region can be matched with the mode value of the TEM.sub.oo mode which is the fundamental of the laser oscillation mode 7. However, it should be noted that the reflectivity of the high reflection multi-layer films of dielectric formed on the first and second side faces 14 and 15 is not more than 99.5% for instance because the manufacturing accuracy is limited; that is, the light suffers from a loss of 0.5% every reflection. Hence, if the number of times of reflection is increased to obtain a high output, then the loss is increased as much.
On the other hand, U.S. Pat. No. 4,785,459 has disclosed a solid laser comprising a diode bar made up of a plurality of semiconductor lasers arranged at certain intervals. FIG. 5 shows one example of the high efficiency mode harmonic type solid laser device disclosed by the U.S. Pat. No. 4,785,459.
In FIG. 5, reference numeral 17 designates a laser block of solid laser medium; 18, a diode bar made up of a plurality of semiconductor lasers arranged at predetermined intervals; 20, a fiber lens; 21, a first incident end face; and 22, a second incident end face.
The operation of the solid laser thus constructed will be described. The output light of the semiconductor lasers forming the diode bar 18 is larger in the divergent angle perpendicular to the surface of the drawing than in the divergent angle in parallel with it. Therefore, the output light is applied to the laser block 17 in such a manner that its vertical component is converted into a parallel light beam by the fiber lens 20, and its beam diameter is harmonic with the size of the lateral mode of the solid laser. If, in this case, the wavelength of the output light of the diode bar 18 is made coincident with the absorption band of the solid laser medium, then the output light of the diode bar 18 is absorbed exponentially while propagating in the laser block 17, so that an inverted population is formed which has gain with the wavelength of oscillation of the solid laser. The inverted population is large at the light emission positions of the semiconductor lasers and small at the connecting positions of the semiconductor lasers, reflecting the spatial distribution of the output light of the diode bar 18 which has passed through the fiber lens 10. A high reflection mirror 4 and an output coupling mirror 5 are so positioned as to form a laser resonator having an optical path such that, inside the laser block 17, the output light of the diode bar 18 is reflected zigzag between the first and side face 14 substantially in coincidence with the light emission positions of the semiconductor lasers forming the diode bar 18 and the second side face 15. A dichroic film is formed on the fist side face 14 to which the output light of the diode bar 18 is applied, which film shows no reflection for the wavelength of the output light of the diode bar 18 but shows high reflection for the wavelength of oscillation of the solid laser. A coating which is highly reflective for the wavelength of oscillation of the solid laser is formed on the second side face 15. A coating which is not reflective for the wavelength of oscillation of the solid laser is formed on the first and second incident end faces 21 and 22 through which the laser beam is applied to the high reflection mirror 4 and the output coupling mirror 5. With the laser resonator thus constructed, the energy of the light emergent from the diode bar 18 can be coupled to the laser oscillation mode with high efficiency. The oscillation output of the solid laser can be increased by increasing the number of semiconductor lasers forming the diode bar 18; i.e., by increasing the number of times of zigzag reflection.
As was described above, the conventional side light excited solid laser shown in FIG. 1 is disadvantageous in that almost all the energy accompanying the output light of the semiconductor laser array 2 is applied to the region of the solid laser medium 1 except that occupied by the laser resonator mode; that is, a larger part of the energy does not employed for amplification of the laser oscillation mode.
In the conventional end light excited solid laser as shown in FIG. 3, the excitation region can be matched with the mode volume of the TEM.sub.oo mode, and the excitation efficiency is high accordingly. However, the solid laser is still disadvantageous in that its output is limited; that is, it is impossible to provide high power because the output of a conventional semiconductor laser is not more than 1 W, and the energy used by the end light excited solid laser is limited even if a higher output semiconductor laser is employed as pumping light source.
The conventional semiconductor laser excited solid laser device as shown in FIG. 4 or 5 is disadvantageous in the following points: (1) In order to realize the excitation with high efficiency, it is necessary to coincide the positions of the semiconductor lasers forming the diode bar 18 with the reflecting positions of the zigzag optical path forming the laser resonator. Hence, it is difficult to adjust the positional relation between the high reflection mirror 4 and the output coupling mirror 5. (2) The high reflection film formed on the laser block 17 is the multi-layer film of dielectric; however, it is difficult to provide a multi-layer film of dielectric having a reflectivity of 100 % because of the optical absorption and scattering of the multi-layer film and the limitation in manufacturing accuracy of the latter; that is, the reflectivity is not more than 99.5%. Accordingly, the light suffers from a loss of about 0.5% every reflection. That is, when the zigzag optical path is increased in the number of times of reflection, the loss in the laser resonator is increased. (3) It is necessary to apply a coating non-reflective for the wavelength of oscillation of the solid layer on part of the side face which is made up of the first and second incident end faces 21 and 22 confronting the end face to which the output light of the diode bar 18 is applied, and the second side face 15, so that the laser beam is applied to the high reflecting mirror 4 and the output coupling mirror 5. That is, in manufacturing the solid laser, it is necessary to define the solid laser with the high reflection coating region and the non-reflection coating region. Thus, the number of manufacturing step is relatively large.